1. Field of the Invention
The invention relates to a device for recognizing ferromagnetic foreign bodies, in particular for protecting the working members of a harvesting machine, e.g. a field chopper.
2. Background Information
A plurality of similar electrical devices for this purpose are known, in particular from DE 22 52 595 A1, DE 24 30 147 A1 and from the magazine "Agrartechnick International", Apr. 1983, page 17. These devices have the common feature that the lines of flux extend in the direction of transport. At least two magnetic poles are seated one behind the other in this direction. This produces a wide construction which leads to difficulties when fitted into a roller and which limits the range of the magnetic fields.
A device of the same generic type is known from EP 02 29 991 B1. For the generation of the magnetic fields, block-shaped permanent magnets are lined up on a flat rail, disposed in the interior of a transport roller and displaying good magnetic conductivity, which permanent magnets are magnetized perpendicularly to the rail and approximately radially in relation to the roller axis. The magnets on half the rail length, i.e. on half the transport path width, form north poles and the magnets on the other half form south poles. The magnetic lines of flux thus extend transversely to the direction of transport. The central magnets, respectively, of the two magnet assemblies are surrounded in each case by a pick-up coil, which pick-up coils--the direction of winding being the same--are connected up in opposition to one another.
An adverse effect in this case is that where electrical conductors are moved closely over the magnet, e.g. in the case of the metallic roller shell, the sensitivity of the device can be substantially curtailed because the eddy currents produced in the electrical conductors then induce disturbance voltages in the pick-up coil. The sensitivity of the device is also negatively influenced by the opposite magnetic polarity at the sides of the transport path. This gives rise to a closure of the magnetic field over the magnetic frame of the harvesting machine, so that moving parts of the frame or their natural vibrations cause disturbance voltages. Finally, the size of the spatial area above the device, within which vagrant ferromagnetic parts are recognized, also does not yet meet the requirements in respect of large harvesting machines. In particular, the height of this "recognition area" is limited due to the large distance between the magnetic poles.